The present invention relates to a process for coating the flow channels in a monolithic, cylindrical catalyst carrier with a coating dispersion.
Monolithic, catalyst carriers are used on a large scale for the production of automotive vehicle exhaust gas catalysts used for pollution abatement. They have a cylindrical shape and a large number of flow channels for the exhaust gases from the internal combustion engine passes through them, the channels lying parallel to the axis of the cylinder. These carriers are frequently also called honeycomb carriers.
The cross-sectional shape of the catalyst carrier depends on how and where it is to be physically incorporated into the vehicle. Catalyst carriers with a round cross-section, an elliptical or triangular cross-section are widely used. The flow channels generally have a square cross section and are arranged in a tight grid over the entire cross-section of the catalyst carrier. Depending on the actual application, the channel density, or cell density, of the flow channels is between 10 and 120 cmxe2x88x922. Catalyst carriers with cell densities of up to 250 cmxe2x88x922 or more are under development.
For the purification treatment of vehicle exhaust gases, catalyst carriers which have been obtained by the extrusion of ceramic materials are used. As an alternative, catalyst carriers made from corrugated and rolled-up metal foils are available. Currently, catalyst carriers with cell densities of 62 cmxe2x88x922 are still mainly used for exhaust gas treatment in private cars. The cross-sectional dimensions of the flow channels in this case are 1.27xc3x971.27 mm2. The wall thicknesses in these kinds of catalyst carriers are between 0.1 and 0.2 mm.
In order to convert the harmful substances present in automotive vehicle exhaust gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds very finely divided platinum group metals are generally used, the catalytic effect of which can be modified by compounds of base metals. These catalytically active components have to be deposited onto the catalyst carrier. However, it is difficult to ensure the requisite very fine distribution of.catalytically active components by depositing these components onto the geometric surfaces of the catalyst carrier. This applies equally to both non-porous metallic and porous ceramic catalyst carriers. A sufficiently large surface area for the catalytically active components can only be provided by applying a support layer consisting of finely divided, high surface area materials.
The present invention provides a process for applying this type of support layer to the internal surfaces of the flow channels of honeycomb-shaped catalyst carriers. In the context of this invention, the support layer for the catalytically active components is called a dispersion coating. The dispersion coating consists of finely divided, high surface area materials and is produced using a so-called coating dispersion. The coating dispersion is a slurry of the finely divided materials, generally in water.
Various processes for depositing the coating dispersion on the catalyst carriers are known from the prior art. After the coating procedure, the catalyst carriers are dried and then calcined in order to consolidate the dispersion coating. The catalytically active components are introduced into the dispersion coating by impregnating with, generally, aqueous solutions of precursor compounds of the catalytically active components. As an alternative, the catalytically active components may be added to the coating dispersion itself. Subsequent impregnation of the final dispersion coating with catalytically active components is not required in this case.
GB 1 515 733 describes a coating process for ceramic catalyst carriers. The porous catalyst carriers are inserted upright, that is with the flow channels in a vertical alignment, into a pressure-resistant coating chamber and degassed by applying a reduced pressure of 0.84 bar (25 inches of mercury). Then the coating chamber is filled with coating dispersion to above the upper end face of the catalyst carrier and this is forced into the pores of the catalyst carrier by applying a pressure which is greater than atmospheric. After reducing the pressure back to atmospheric and opening a discharge valve in the base of the coating chamber, excess coating dispersion flows out of the flow channels in the catalyst carrier. Then any flow channels which are blocked with coating dispersion are blown clear from top to bottom using compressed air. The cycle time for this coating process is from less than 1.5 to 2 minutes.
U.S. Pat. No. 4,208,454 also describes a process for coating porous ceramic catalyst carriers. The lower end faces of the catalyst carriers to be coated are placed on the opening of a collection vessel in which the pressure is reduced to 5 to 16 inches of water below atmospheric pressure by means of a large volume fan. This reduced pressure is held constant during the entire coating period. A predetermined volume of coating dispersion is distributed over the upper end face of the catalyst carrier and drawn uniformly through the flow channels into the collection vessel. The suction process is maintained for at least about 30 seconds. After the first 5seconds the entire amount of coating has been drawn through the catalyst carrier. During the remainder of the time the air flowing through the flow channels ensures that any flow channels blocked by coating dispersion are cleared. The amount of coating remaining on the catalyst carrier can be affected by the duration of the total suction time and by the extent to which the pressure is reduced. Axial uniformity of the coating on the catalyst carrier can be improved by turning the catalyst carrier over after about half the suction time and applying the suction in the reverse direction. Using this process, coating dispersions with 30 to 45% solids contents and a viscosity between 60 and 3000 cps can be processed. The preferred solids content is 37 wt % and the preferred viscosity is 400 cps. The reproducibility for the amount of coating applied using this process is given as xc2x15%.
EP 0 157 651 B1 also describes a process for coating ceramic catalyst carriers with a predetermined amount of a coating dispersion. Here, the pre-weighed amount of coating dispersion is placed in an open, wide, vessel and the lower end face of the catalyst carrier is immersed in the dispersion. Then the dispersion is drawn into the flow channels of the catalyst carrier under suction, by applying a pressure which is slightly below atmospheric to the upper end face. To improve axial uniformity of the coating, it is also recommended here that the coating process be allowed to proceed in two steps.
In the first step, only about 50 to 85% of the total amount of coating is placed in the vessel and drawn into the catalyst carrier under suction. Afterwards, the catalyst carrier is turned over and the remainder of the coating is drawn into the catalyst carrier under suction in the reverse direction. This coating process does not require a separate step for clearing any blocked flow channels. The cycle time for this process is somewhat less than 1 minute. Using this process, coating dispersions which have a solids content between 35 and 52% and viscosities between 15 and 300 cps can be processed.
U.S. Pat. No. 5,182,140 describes a process for coating ceramic and metallic catalyst carriers. In this case, the coating dispersion is pumped from below into the vertically arranged catalyst carrier until the dispersion reaches a height which s well above the upper end face of the catalyst carrier. Then excess coating dispersion is removed from the carrier applying compressed air to the upper end face of the catalyst carrier. This simultaneously blows out any flow channels which are still blocked. In accordance with example 1 in this patent document, the coating dispersion is adjusted to reach an ultimate height of 2 cm above the upper end face of the catalyst carrier. The compressed air for blowing out excess coating dispersion from the flow channels is supplied in two consecutive pressure stages. During the first 2 seconds after filling the catalyst carrier, the coating dispersion is subjected to compressed air at 3.7 bar. This high, pressure means that excess coating dispersion is completely blown out of the flow channels during the available 2 seconds. Then the pressure of the compressed air is reduced to 0.37 bar and the catalyst carrier is subjected to this pressure twice, for 0.5 seconds each time. With this process, coating dispersions which have a specific density between 1 and 2 g/ml and a viscosity between 100 and 500 cps can be processed.
DE 40 40 150 C2 also describes a process for uniformly coating a honeycomb carrier made of ceramic or metal. Here, the honeycomb carrier is introduced into an immersion chamber and filled from below with coating dispersion. Then the honeycomb carrier is emptied by blowing or by suction. The honeycomb carrier is then taken out of the immersion chamber and excess dispersion is removed by suction or by blowing, in order to avoid blocked flow channels, in a separate unit. Using this process, coating dispersions with solids contents between 48 and 64 wt. % and viscosities between 50 and more than 100 cps can be processed.
The processes described are suitable for the coating of both ceramic and metallic carriers. In the case of metallic carriers consisting of stacks of metal strips, DE 4233404 C2, WO 92/14549, and EP 0775808 A1 disclose that the metal strips are coated in a strip coating unit before being assembled into the carrier, as an alternative to coating the final carrier.
The treatment of exhaust gases from internal combustion engines is subject to increasingly stringent Federal and State legal requirements with regard to conversion of the harmful substances. In order to comply with these requirements, catalyst carriers with higher and higher cell densities are being developed. The greater number of catalyst carriers produced, however, still has cell densities of only 62 cmxe2x88x922. A small number of carriers with cell densities of 124 cmxe2x88x922 has been manufactured. These are mainly carriers made from metal strips.
Carriers with cell densities of more than 186 cmxe2x88x922 are under development. In addition, attempts have been made to improve the conversion of harmful substances using so-called start catalysts which are incorporated into the exhaust gas pipe close to the engine, upstream of the actual main catalyst. These are small volume catalysts which may also have high cell densities. These catalysts can also be used to advantage for treating exhaust gases from motor cycles.
The coating processes described for catalyst carriers are suitable to only a limited extent for the coating of small volume catalyst carriers. This is true in particular in the case of small volume catalyst carriers with high cell densities. The cycle rates which can be produced with known processes are too small for an economically viable coating process. Only large carriers with low cell densities can be coated effectively using these processes. Monitoring the viscosities of coating dispersions is sometimes costly because the coating dispersion loses a considerable proportion of its moisture content due to prolonged contact with the air streams used to clear the flow channels and this has to be continuously topped up in order to be able to ensure reproducibility of the coating.
The production of small volume catalysts from previously coated metal strips, on the other hand, leads to losses of active coating material due to blockage of the flow channels when assembling the catalysts. These losses may amount to up to 10% in unfavourable cases. In addition, it is a characteristic of this mode of operation that acute-angled cavities are formed at the contact points between neighbouring metal strips and this has an unfavourable effect on the access of exhaust gases to the catalytic coating and thus reduces the catalytic activity of the catalyst.
Therefore, an object of the present invention is to be able to coat honeycomb-shaped ceramic and metallic catalysts in a cycle time of less than 10 seconds and to enable the reproducible coating of carriers with a cell density of more than 180 cmxe2x88x922.
The above and other objects of the invention can be achieved by a process for coating the flow channels in a monolithic, cylindrically shaped, catalyst carrier with a coating dispersion, wherein the carrier has two end faces which are connected to each other via flow channels arranged parallel to the axis of the cylinder. Coating is performed by vertically aligning the axis of the cylindrical carrier, placing a predetermined amount of coating dispersion from a storage container on the upper end face of the carrier and drawing the dispersion through the flow channels under suction, removing excess coating dispersion from the flow channels by emptying the flow channels under suction, returning the excess dispersion to the storage container and fixing the dispersion coating by calcination.
The process is characterised in that the coating dispersion is drawn through the flow channels under suction at a rate of flow of 0.1 to 1 m/s (meters per second) and that after completion of passage under suction the excess coating dispersion is removed from the flow channels by applying a suction impulse from below, wherein the suction air is drawn through the flow channels under suction at a rate of flow between 40 and 1 m/s and the excess coating dispersion discharged with the air stream is separated from the air stream within a time of less than 100 ms after discharge from the catalyst carrier.
According to the invention, therefore, coating of the flow channels is performed in two stages. In the first stage, a predetermined amount of coating dispersion is placed on the upper end face of the catalyst carrier and drawn through the flow channels under suction at a rate of flow 0.1 to 1 m/s by applying a pressure which is lower than atmospheric to the lower end face. The predetermined amount of coating dispersion is preferably such that it corresponds to 0.5 to 2 times the free volume of the flow channels. The rate of flow is advantageously chosen so that the drawing through under suction process is complete after less than one second.
The second stage follows immediately after this first stage and in this stage the flow channels are cleared of excess coating dispersion by applying a suction impulse. Here, a suction impulse is understood to be a process in which initially a very large amount of air is conveyed through the flow channels. During the course of the suction impulse, however, the amount being conveyed decreases continuously.
This type of suction impulse can be achieved by connecting the lower end face of the carrier to a large container at a pressure which is less than atmospheric, which results in the initial rate of flow of suction air in the flow channels being very high and then decreasing continuously during the course of the suction process since the difference between the pressure in the reduced pressure container and atmospheric pressure is continuously reduced by the suction air being conveyed thereto. According to the invention, the initial rate of flow of the suction air should be between 5 and 40 m/s. At the end of the suction process the rate of flow is reduced to the minimum rate of about 1 m/s.
During the first stage of the process the lower end face of the catalyst carrier is also connected to the reduced pressure container in order to draw the coating dispersion through the flow channels under suction. However, a suitable flow restrict or has to be used to ensure that the rates of flow according to the invention of coating dispersion are maintained in the flow channels.
An essential aspect of the invention is the early separation from the suction air of the excess coating dispersion discharged from the flow channels with the suction air. This reduces the extraction of liquid from the coating dispersion and simplifies recycling of the excess coating dispersion to the storage container. Without this measure, the solids content of the coating dispersion in the storage container would constantly increase and thus make reproducible coating of the catalyst carriers difficult.